8 



AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



2. Structure (Fig 1, A). An outer perfectly transparent 

 exoplasm can be distinguished from a more granular internal endo- 

 plasm. The exoplasm is apparently stiffer than the endoplasm, 

 but there is no clear line of demarcation, and as part of the body 

 may be outside one moment, and inside next, the apparent differ- 

 ence is probably caused by a tendency of the granules to collect 



Fig. 1. AM(EBA AND VoRTiCELLA. (from ffaddon, after Howes, and Stein], 

 enlarged. A-0, Stages in fission of Amoeba; D and E, fission of other 

 Protozoa; F, conjugation of Vorticella. c.v., Contractile vacuole ; 

 n, nucleus. [ 



in the centre. Some amoebae, however, possess a firmer exoplasm 

 in which a sort of fibrillation can be made out. 



Within the granular portion two important structures are 

 present: (1) The nucleus (n), a rounded or ovoid mass, consist- 

 ing of a modification of protoplasm, and denser than the rest of 

 the body. In the living animal it is inconspicuous, but becomes 

 very obvious on treatment with weak acid or a staining solution. 

 In some cases, at any rate, the nucleus is invested by a delicate 

 membrane, and consists of two substances, one, chromatin, staining 

 readily the other, achromatin, staining with difficulty. The 

 arrangement of the chromatin differs with the species, but part 

 of it is frequently aggregated into a central particle, the nudeolus. 

 (2) The contractile or pulsating vacuole (c.v.), a spherical space 



